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The multiple challenges involved in flash LED driver selection–and how to solve them

In the cellular handset market, the inclusion of high pixel count imagers integrated into the handset is almost standard. As the resolution of these imagers increases, the need for brighter flash sources also increases. The use of Xenon flash bulbs has long been the main illumination choice in digital cameras. In the mobile handset board area for non-telephone related functions is sparse and the Xenon approach is often impractical due to solution size. Luckily for handset manufacturers, recent advancements in high power white light emitting diodes (LEDs) have been dramatic. Manufacturers of white LED flash diodes now have products that can deliver light outputs greater than 70 lumens, and can handle pulsed currents greater than or equal to 1A. Along with these advancements come new questions the handset designer must answer. How much PCB area can be used? What added flash related features are needed? How much power can the flash driver be allowed to use? How much illuminance is needed to take a decent picture? Answering these questions can greatly aide the designer in the selection of a flash LED driver.

Solution Size
One of the first questions a handset designer must ask is how much board area can be devoted to a camera flash? In the LED flash driver arena, two boost technologies are commonly used: Switched Capacitor Boost (Charge Pump) or Inductive boost. Of the two boost topologies, the switched capacitor implementation is typically smaller. Most switched capacitor parts require four ceramic capacitors and two external resistors. The typical capacitor value recommended for these applications is 4.7uF with a voltage rating of 10V (to help with DC bias losses). These capacitors can be found in 0603 cases sizes from a number of capacitor manufacturers. Total solution sizes at or around 25mm2 are fairly common when utilizing switched capacitor flash drivers. Some switched capacitor flash drivers come in a chip-scale package and can have a total solution size that is less than 15mm2 . Switched capacitor solutions also have the advantage of being very thin. Depending on the flash driver package, the capacitors are often the tallest component in the solution.

Inductive flash drivers tend to have larger solution sizes compared to the switched-capacitor driver. A typical solution size for an inductive flash LED driver is closer to 35 to 40mm2 of board area. Inductive drivers typically require two capacitors (input and output) with average capacitor values of 10 μF and are available in 0805 case sizes. Inductive boosts require a rectification element that can handle the peak inductor current and output voltage. In a synchronous boost, a pass FET (typically a PFET) is usually integrated into the flash IC. This integration often causes the size of the IC package to increase over an asynchronous solution. In an asynchronous topology, the pass element takes the form of a schottky diode. The main area increase of the inductive boost compared to the switch capacitor boost is the inductor itself. Applications with flash currents approaching 1A, typically require a 2.2-4.7 μH inductor with a saturation current greater than 1.5A. These inductors rarely can be found in footprint smaller than 3mm x 3mm. The inductor also tends to be the tallest component in the solution. 1.2mm heights are not uncommon.

Features
Once the topology of the flash driver has been decided upon, the next question to ask pertains to the feature set required for the design. The first feature to address is the type of control interface. The basic flash driver usually has two control pins allowing for three to four modes of operation (ex. Shutdown, Indicator, Torch and Flash). If the designer does not need to change brightness levels dynamically, these simple control parts are usually sufficient. On the other hand, if a higher level of control is desired, many flash ICs have some sort of serial control interface. One of the most popular serial interfaces used is the Inter-Integrated Circuit interface (I2 C). The I2 C or I2 C compatible interface not only controls the basic on/off functionality, but also allows the user to set the torch and flash levels dynamically. Other features such as the flash safety timer duration, inductor current limit level, and over-voltage protection level, if present, can be configured through the I2 C interface. These serial interfaces are especially useful when the general purpose input/output (GPIO) lines of the microcontroller/microprocessor are limited.

Many LED flash drivers provide additional control pins to further aid the designer address system level concerns. Today's imagers typically have an external strobe/flash pin that alerts the system that a picture is being taken. This strobe/flash signal can be tied to many LED flash drivers directly via a flash enable pin. A direct connection between the imager and LED driver eliminates any delay that could occur between the two parts due to controller/software limitations.

One of the largest system level issues in the handset today is managing the amount of current drawn from the battery during a call/data transmission. The combination of the current drawn by the Tx/Rx power amplifier during a call/data transmission and the current drawn by the flash driver can exceed the maximum allowed battery current. Most handset designs allow operation to occur down to a loaded battery voltage of 3.2V before the handset goes into a reset state (VBATT-LOADED = VBATT_UNLOADED ” (IBATT * RBATT_ESR) ). To prevent a reset condition caused by the ESR voltage drop in the battery, some of the newer flash LED drivers have incorporated a transmission pin (Tx) to help minimize the current drawn by the LED driver during a call/data transmission. By asserting the Tx pin on these parts, the flash driver can force the diode current to a lower level in a very short time (less than 100 μsec.) preventing the handset from going into a reset state during a call.

Efficiency
The topic of efficiency is not new to the world of handset design. The higher the system efficiency, the longer the talk time the user can experience. Inductive boosts can yield high efficiencies over large input voltage and output current ranges. Switch Capacitor parts are limited to a few fixed quantized gains (2x, 1.5x, 1x pass mode) resulting in a lower average converter efficiency over the same input range. When evaluating LED flash drivers, the topic of efficiency takes on a slightly different meaning.

Converter Efficiency or LED Drive Efficiency?
When dealing with Flash LED drivers, certain efficiency losses must be considered to obtain the solution efficiency. In order to have a controlled flash or torch/movie light LED current, a boost converter must either utilize a current sink/source or a tightly controlled reference voltage along with a resistor to set the load current. While both solutions have advantages and disadvantages, it is important to remember that both methods introduce a power loss that does not get taken into account when calculating boost converter efficiency. The solution/LED efficiency takes this power loss into account.

Two different converters can have the exact same converter efficiency and have LED drive efficiencies that are 5 to 10% different. In the end, if the converter efficiencies are the same, the converter that introduces the smallest loss due to the current regulation element should be considered the more efficient converter.

LED Drive Efficiency or Luminous Efficacy?
Unfortunately, LED drive efficiency does not tell the whole performance story. For example, let's assume that we have two Flash LED drivers and two different flash LEDs. The first driver has a converter efficiency of 85% and the LED voltage is 4V at 1A of current. The second driver has a converter efficiency of 80% and a LED voltage of 3V at 1A. Both LEDs produce the same amount of light for a given current and both converters have a feedback voltage equal to 350mV. Using Equation#1, at a worst case input voltage or 3.2V, the first driver will draw 1.6A from the battery. Using this same equation, the second driver will only draw 1.3A from the battery. Despite having a higher efficiency, flash driver #1 draws 300mA more current to produce the same amount of light as flash driver #2. This example highlights the effects of the LEDs luminous efficacy. The LED in example #2 is 33% more efficient at producing light than the LED in example#1.

Converter efficiency, while important when flash LED drivers are running in a continuous movie/torch mode due to potentially long on-times, is not as important in the traditional sense during a flash condition due to the short duration. What is important is that higher efficiencies allow a higher amount of output power to be delivered to the flash LED for a given input power, allowing for a brighter flash. Picking a flash LED with a high luminous efficacy in conjunction with an efficient flash LED driver can help minimize the current drawn off of the battery during a flash event, allowing the handset designer more flexibility in the power management scheme for the rest of the system.

Optimizing Light Output
Optimizing the light output of the LED flash driver involves answering two key factors (three if we include cost): How much illumination is required for the handset imager, and how much power can be used to provide the illumination? For a given input power budget, there are three main ways to increase the brightness of a flash event to help the designer reach the illumination target. LED selection, LED current drive, and LED configuration all play a huge role in optimizing the light output of a given flash LED driver.

Picking the LED
The first optimization factor has already been touched upon and involves selecting an LED with a high luminous efficacy (lumens per watt). An LED with a higher luminous efficacy emits a higher amount of luminous flux (lumens) for a given power. To calculate the luminous efficacy, you divide the luminous flux of the LED at a given current by the drive current multiplied by the forward voltage of the LED. Luminous flux curves can be found in most LED manufacturer datasheets.

When picking the LED, the handset designer must not only take into account the optical performance of the LED, but also the size and cost of the LED, and the complexity of the lens required to maximize the illuminance of the LED.

Increase Drive Current
With the LED chosen, the second way to increase the light output is to increase the actual drive current. Using the luminous flux curve in Figure #2, transitioning from a diode current of 500mA to 1A causes an increase of approx 30 lumens. Increasing the diode current does have some drawbacks. Doubling the diode current causes the LED power to increase by more than double due to the increase in current and the increase in forward voltage of the diode. This increase in LED power translates to a higher input power requirement. Figure #3 highlights the effect of forward current on the forward voltage of the LED

To optimize the LED driver current within a system, an input current budget must first be established. Once the minimum input voltage and maximum input current has been decided upon an iterative calculation based upon data found in the LED's forward voltage vs. LED current curve (Figure #3) can help find what the maximum allowed drive current can be for a given flash LED driver.

Example

Looking at the curve, an LED at 3.6V and 1A of current yields an output power equal to 3.95W (PLED + PFEEDBACK) which is very close to the maximum allowed.

Configuration
If optimizing the flash current through an LED with a high luminous flux and luminous efficacy does not produce the desired amount of illuminance, adding a second or third LED to the design will move the design closer to the illuminance target. Referring back to the Luminous Flux vs. LED Current curve (Figure 2), it should be noted that the curve is not purely linear. Two LEDs running with half of the total flash current through each LED produces more luminous flux than one LED at the full flash current. In addition, the total LED power will drop with two LEDs at half the flash current. This is due to the forward voltages of the LEDs reducing with the drop in current. This allows for a higher total output current to be delivered to the two LEDs while staying within the input power budget. These two LEDs can be driven either in a parallel configuration or in a series configuration.

Example:

The series implementation provides numerous advantages over the parallel implementation when driving two LEDs. Driving two LEDs in series ensures that the current through both flash LEDs is equal. In the parallel configuration, two current sources can typically match the LED currents within 1 to 3%. Unfortunately, most flash drivers only have a single current sink. Tying two LEDs to a single current source/sink can lead to extreme levels of current mismatch in the LEDs due to mismatched LED forward voltages. To counteract this issue, the addition of series ballast resistors is required. The addition of a resistive element in series with the LED decreases the output current budget and lowers the amount of usable flash current, causing a dimmer flash event. In addition, driving two LEDs in series lowers the amount of output power dissipated in the current control element (resistor or current sink) by two if the LED currents are equal in both the series and parallel configuration (PFB-Series = ILED – VFB and PFB-Parallel = ILED – VFB – 2).

Summary
Adding a white LED camera flash to a system involves a number of design choices. Determining how much board area a flash driver can occupy will often determine which boost topology can be used. Features such as a transmission pin or flash enable can help ease the stress on the battery and micro-controller/processor by allowing other sub-systems to handle some of the current management and flash timing. Using an optically efficient LED in conjunction with an efficient boost converter can help maximize the illuminance of the flash system. If a single LED does not provide enough illumination to take a decent low-light picture, adding a second LED to the design could help over-come some of the lighting deficiency. Choosing the right LED flash driver can seem like a daunting task if these types of questions are not addressed early in the design cycle. By examining these issues and concerns early, many future headaches can be prevented.

For simplicity, this article will use the luminous flux curves instead of the illuminance curves to compare different LEDs and LED configurations. Illuminance is highly dependant on the size and shape of the lens used in the handset and will differ from manufacturer to manufacturer.

About the Author
Greg Lubarsky is an Applications Engineer with National Semiconductor's Grass Valley design center working in the Mobile Devices Power group. He received his Bachelor of Science degree in Electronic Engineering from the University of California, Davis in 2002. He has over 5 years of experience as an applications engineer working with white LED backlight drivers, LED flash drivers and other power management electronics. He can be reached at: .

For related information see:
National Semi claims first high-current flash LED driver with adjustable over-voltage protection

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